1. Technical Field
The invention relates to a method for determining the spatial distance of two transmitter-receivers communicating without wires from one another within a transmission range in the order of magnitude of 10 km. The invention further relates to a system of at least two transmitter-receivers communicating without wires, designed to determine the spatial distance of the transmitter-receivers from one another within a transmission range in the order of magnitude of 10 km. The invention finally relates to a transmitter-receiver for performing said method.
2. Discussion of Related Art
Three different media are employed to measure distances between mobile and stationary objects. These are: radio, infrared and ultrasound. The so-called pulse transit time method is mainly used for this. The transit time of transmitted pulses is determined by measuring the time of arrival (=TOA) and comparing it with the start time of the pulses. The distance is then calculated from the pulse transit time, also called the signal transit time in the context of this application, at the known propagation speed in the medium.
Another method for measuring distance measures the receive signal strength (=RSS) of received pulses and from this estimates the distance. The receive signal strength is strongly influenced by interference, attenuation and reflections. Experience has shown that this method is too inaccurate and unreliable for measuring distance.
A further method by which the distance can likewise be calculated is determining position by determining the angle of a arrival (=AOA) and triangulation. However, determining the distance of an object requires two cooperating devices at a known distance from one another.
Measuring the angle is relatively complicated (special antennae, time expenditure) and, because of interference, attenuation and reflections, inaccurate. If there is no visual connection, measuring the angle is greatly restricted or no longer possible, depending on the medium used. Experience has proved that this method is also too inaccurate and unreliable for measuring distance.
Infrared and ultrasound methods according to the pulse transit time method likewise have a considerable number of disadvantages. The transmission range of the systems is not very large, normal environmental influences such as daylight, sound sources or reflections interfere with them greatly and signal attenuations alone, e.g. due to thick smoke or haze, lead to the failure of these systems. Owing to said disadvantages these methods are not suitable for precise and reliable measuring of distance.
Pulse transit time methods based on radio communication signals are less sensitive to interference caused by the environment and have proved to have the greatest accuracy.
Classical radar systems are considered disadvantageous as far as expenditure, immunity to interference and unambiguous identification of objects is concerned. Reliability and accuracy depend greatly on the reflective properties of the object to be measured and the propagation conditions of the radar waves.
Also applied, in addition to the pulse transit method, is the method of determining the angle of arrival, which has the already mentioned disadvantages. It is not possible to transmit all types of data with the classic radar systems.
Satellite-aided location systems are widely used systems, which apply distance measurement according to the pulse transit time method for determining positions. These include, for example, the location system known by the name of Global Positioning System (GPS). The satellite-aided systems have a number of disadvantages. Set up and operation of the infrastructure is very complicated and expensive. Availability is greatly reduced by attenuation where there is restricted visual connection to the satellites (e.g. narrow gorges, woods) and no longer provided in shadow (e.g. in buildings). Moreover, only determining their own position is possible, but not determining the position of remote objects. In order to determine the distance from a remote object, an additional system is required to transmit messages, as this is not possible via the satellites. Only by a second position determination and exchange of the position data is measuring the distance possible. The accuracy is additionally not estimated to be sufficiently precise, since the errors of two position determinations may accumulate in the distance calculation (1-3 m with differential GPS). Furthermore, the equipment for determining position is distinguished by over-long measuring times of up to several minutes, making determining the distance of mobile objects too inaccurate or no longer possible. Transmission of all types of data, as already mentioned, is not possible via the satellite-aided systems. In addition, there is a dependency on the operator who may restrict the accuracy and reliability and charge fees for usage.
A further option for measuring distance is determining the position of mobile radio subscribers with the aid of mobile terminals, e.g. mobile telephones for operation according to the GSM standard. However, the attainable accuracy of determining position of at best 60 m is regarded as very disadvantageous. Here too there is dependency on the operator, who nearly always charges fees for usage.
Further known methods and systems which measure the round time of a sequence of radio pulses between two transmitter-receivers (active pulse radar) also have a number of disadvantages. In the known round time methods on the TOA principle (time of arrival), also called two-path method, there is a very great dependency of the accuracy of determining the position on the accuracy of the clock generators for measuring time. With simultaneous transmission of data, at purely the pulse transit in the medium, relatively long round time intervals arise conditional on the transmission time of the data frames. A typical ratio is 1:10,000. Very precise clock generators with an accuracy of more than ±1 ppm and therefore cost-intensive components are necessary to achieve high accuracy. If reasonably priced clock generators (±50 ppm) were to be used, only inadequate accuracy (c. 10 m) would be achievable.
Increasing accuracy is for this reason also achieved by shortened round time intervals. However, the result of this is that fewer to no additional data at all can be transmitted and special message protocols have to be used. Standardised message protocols cannot be used, which is a further considerable disadvantage of these methods in relation to recyclability or the capacity to extend standards.
The achievable accuracy with the known methods is, moreover, dependent on the smallest unit of the time measurement. Therefore, to increase accuracy, very small time units are used, making necessary the use of very fast clock generators and time counters (30 cm distance corresponds to 1 ns time unit or 1 GHz clock frequency). Time counters faster in this way are regarded as very disadvantageous, though, as this is accompanied by an increase in energy consumption and costs of implementation.